Question

I am trying to convert a PHP code to python.

All values are send in network byte order (big endian).

Basically, the REQUEST in protocol specification is

and response is

Corresponding PHP Code (corresponding DOC) is:

$transaction_id = mt_rand(0,65535);
$current_connid = "\x00\x00\x04\x17\x27\x10\x19\x80";
$fp = fsockopen($tracker, $port, $errno, $errstr);
$packet = $current_connid . pack("N", 0) . pack("N", $transaction_id);
fwrite($fp,$packet);

I am trying to find the corresponding code (for doc) in python:

transaction_id = random.randrange(1,65535)
packet = "\x00\x00\x04\x17\x27\x10\x19\x80"
packet = packet + struct.pack("i", 0) + struct.pack("i", transaction_id)
clisocket =  socket.socket(socket.AF_INET, socket.SOCK_DGRAM) 
clisocket.sendto(packet, ("tracker.istole.it", 80))

In the response, I should get the same transaction_id I sent in the request which I am not getting. So, my guess is, I am not packing using correct format.

Also, the python documentation is not as clear as that of PHP. The protocol specifies to use Big Endian format & PHP doc clearly states which are the ones for Big-Endian.

Sadly, I could not comprehend which format to use in python. Please help me in choosing the corrent format.

EDIT:
Not getting any responses, so I would say more.

import struct
import socket
import random

clisocket =  socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
packet = "\x00\x00\x04\x17\x27\x10\x19\x80"
transaction_id = random.randrange(1,65535)
print transaction_id
packet = packet+struct.pack(">i", 0)
packet = packet+struct.pack(">i", transaction_id)

clisocket.sendto(packet, ("tracker.istole.it", 80))
res = clisocket.recv(16)

print struct.unpack(">i", res[12:16])

According to protocol specification, I should be returned same INTEGER.

Answer 1

The php pack function format N means unsigned 32-bit big-endian integer.
The corresponding Python struct.pack format is >L.

The images you posted for the protocol show the connection_id should be 64-bit (unsigned) integer: Python struct.pack format Q.

So:

clisocket = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
connection_id=0x41727101980
action=0
# transaction_id = random.randrange(1,65535)
transaction_id = 12345    
print(transaction_id)
# 12345

packet=struct.pack(">QLL",connection_id,action,transaction_id)
print(repr(packet))
# "\x00\x00\x04\x17'\x10\x19\x80\x00\x00\x00\x00\x00\x0009"

clisocket.sendto(packet, ("tracker.istole.it", 80))
res = clisocket.recv(16)
action,transaction_id,connection_id=struct.unpack(">LLQ",res)
print(action)
# 0
print(transaction_id)
# 12345 
print(connection_id)
# 2540598739861590271